Hello, thank you very much for your answer!
On my side, I made a fork of the driver and fixed it myself.
I would like to point out that the problem is not only with the AsynchronousTlsChannelGroup class, but also with the ServerTlsChannel, TlsChannelImpl, DirectBufferDeallocator and Util classes which were using the SLF4J logger directly. 
Have a nice day!